A polyamide film is widely applied to the field of various wrappings which make use of its characteristics such as toughness, impact resistance, proof against pinholes, barrier property against oxygen and the like. In recent years, the demand for a polyamide film has increased with the growth of retort foods so that it is desirable to provide a film having a high quality and a low price.
Methods of preparing a polyamide film conventionally include an inflation method and a melt extrusion method by means of a T-die method.
In the case where a polyamide film is obtained by the T-die method, a molten film extruded through a die is cast onto a revolving cooling or chill drum. At this time, in order to bring the film and the revolving cooling drum into close contact, certain methods are carried out according to which air is sprayed through an air-knife (hereinafter referred to as the "air-knife method"), a charge is induced on the molten film to contact the film by static electricity (hereinafter referred to as the "electrostatic pinning method"), and the like. However, casting with such an air-knife method and electrostatic pinning method traps air between the revolving cooling drum and the film by reason of an accompanying air current which is caused by the revolutions associated with a high haul-off speed so that a uniform film can not be obtained.
The electrostatic pinning method, which is, for example, disclosed in Japanese Patent Publication No. 6142/1962, is a method wherein an electrode, for example, which is of the wire type, is inserted between a die and a cooling drum to provide a static charge for a film article, so that the film article is closely contacted onto the cooling drum with static electricity whereby uniformity of thickness and transparency of a formed film are improved. According to such a method, a molten extruded film article is closely contacted onto the cooling drum by static electricity to be cooled, and hence, there can be efficiently obtained a thermoplastic resin film.
Further, as to a polyamide resin, there is disclosed a cooling method for a polyamide type thermoplastic polymer film which is characterized by subjecting a film to corona discharge at a condition of streamer corona discharge, inducing a charge on a molten film and contacting it closely onto a revolving cooling drum with static electricity (cf. Japanese Patent Laid Open Publication No. 17559/1980). It is also known that this method is successful for a synthetic resin having a melt specific resistance of not more than 6.0.times.10.sup.6 .OMEGA. cm (cf. Japanese Patent Laid Open Publication No. 105930/1981).
However, such a method has also insufficient adhesion when the angular velocity of the cooling drum is raised for increased production efficiency, so that air is trapped between a sheet and the revolving cooling drum at a partial area. At that area, the heat transfer between the cooling drum and the sheet is decreased producing inequality of cooling effect among sufficient contact areas and insufficient contact areas so that uniformity of thickness and transparency of a sheet article becomes remarkably worse thereby damaging a value of the article. Moreover, when a sheet is subsequently subjected to uniaxial or biaxial orientation, it may be impossible to produce a film or a sheet itself because of breakage of a sheet in the stretching stage.